Replacement of coal products by charcoal in metallurgical, chemical and other industries has many advantages, particularly in regard to the improvement of product quality and environmental benefits. However, the current use of charcoal by industry is limited by its relatively high price, which is mainly due to the cost associated with its production as well as the cost of raw materials. Current charcoal production technologies cannot produce charcoal at prices that are competitive with coal products in most economic environments throughout the world. As such, there is a clear need to develop a new and economically viable method of charcoal production, which is also capable of utilising low-grade ligno-cellulosic material.
The main factors affecting the price of charcoal include: capital costs of plant installation, the type of process (i.e. batch or continuous), the scale of the reactor, the residence time required by material in the reactor, maintenance costs, the cost of raw materials, the energy efficiency of the process and the value retained with by-products.
It is preferable to use wood chips/waste, rather than quality timber logs, as a raw material. While use of quality wood generally results in a high quality charcoal product, use of wood chips and waste is more economically viable and cost effective.
Furthermore, the use of wood chips and wood waste decreases the residence time in a reactor, in comparison with wood logs, as the heating time for wood chips is shorter. However, with the use of wood chips/waste heat transfer to the core of a large mass of wood chip/waste material is slow due to the bulk material possessing low gas permeability and low thermal conductivity. Thus neither heating, by blowing through a hot gas nor externally is efficient for large volumes of material.
There are many large scale commercial processes for producing charcoal. Some of the main processes include the Lambiotte/Lurgi retort system (U.S. Pat. No. 2,289,917), tubular reactor systems with material driven by a screw e.g. Thomsen Retort (U.S. Pat. No. 3,110,652), rotating tube retort systems e.g. Seaman retort (U.S. Pat. No. 1,115,590), multiple heath furnace systems e.g. the Herreshoff furnace (Handbook of charcoal making. Solar Energy R&D in the European Community. Series E, Energy from biomass, v. 7 (1985)), fluidised bed pyrolysis reactors, and the Badger-Stafford process (Nelson, W. G. Waste Wood Distillation by the Badger-Stafford Process. Ind. Eng. Chem. (1930). No. 4, Vol. 22, pp. 312-315.).
Many of these systems have limitations which prevent synthesis of charcoal at the desired quality for an economically viable price, that is competitive with the cost of coal.
FIG. 1 shows an embodiment of one charcoal production technique known as the Stafford process (U.S. Pat. No. 1,380,262). Stafford found that when wood chips are bone dry (with a moisture content of less than 0.5%) and pre-heated to at least 150° C., the thermal decomposition of sufficiently large masses of material can be fully autogenous even in the oxygen-free atmosphere (U.S. Pat. No. 1,380,262). Thus, in comparison to other processes, neither the blowing of hot gasses through the material nor the external heating is needed in order to conduct pyrolysis. In the context of the invention, the term “autogenous” is used herein to describe a process, which spontaneously generates a sufficient amount of heat to be self-sufficient in an oxygen-free atmosphere.
The Stafford process is preferably conducted in a vertical cylindrical continuously operating retort, in which the ingress of gas is prevented during loading and extraction of materials (U.S. Pat. No. 1,380,262).
Wood in the cooler zone of the retort is heated by pyroligneous vapours and gases ascending from the hotter zone. The wood is heated to a temperature corresponding to the point at which the carbonisation reaction becomes vigorously exothermic (this occurs at a temperature of approximately 300° C. for wood). Even limited gas permeability is sufficient for this heating mechanism to proceed, as the overpressure of vapours evolved in the hot zone pushes them towards the gas/vapour outlet located at the top of the reactor. The maximum temperature reached in the Badger-Stafford process is approximately 515° C.
Due to the exothermic nature of the process, once the process is operating heat is only required initially to dry the wood and preheat it to 150° C. prior to the wood entering the retort. Charcoal leaves the retort at approximately 255° C. and is transported straight into a charcoal conditioner, which is a rotating tube with water cooled walls.
The Badger-Stafford process is able to convert wood chips/waste, but not sawdust into charcoal, as maintaining minimum required level of the gas permeability of material is important. The smallest wood pieces that can be processed are approximately 2 mm×2 mm×50 mm in size; the estimated residence time for material in the reactor ranges from between 1.5 to 3 hours.
Whilst the Badger-Stafford process offers advantages over the other previously mentioned methods of charcoal synthesis, it also has several shortcomings.                There is no heat recovery from the charcoal as it is cooled i.e. the energy efficiency of the process can be improved.        The process has limited flexibility and controllability i.e. there are no means to increase the temperature to substantially above 515° C. or to reliably maintain a lower temperature, e.g. at 450° C., if required.        There is no efficient control of the heating rate and residence time for every portion of material within the reactor.        Vapours migrate upwards from warmer layers to cooler layers of material and are then extracted, as a result some valuable high boiling point fractions are condensed within these cooler layers of material and cannot be extracted from the reactor.        As organics escape the retort, if maximising the charcoal yield is desirable, there is no opportunity to recycle the organics into the reaction zone to increase the charcoal yield.        
The present invention provides an apparatus for the synthesis of charcoal and offers improvements over the Badger-Stafford process. In particular the present invention allows at least one of improved flexibility and controllability of the process, expanded scalability of the process, improved energy efficiency, increased productivity of the reactor, increased charcoal yield, improved quality of liquid products and faster conditioning of charcoal.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgement or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.